This invention relates to a process for making phosphorous halides and oxyhalides from phosphorus-containing byproducts. In particular, it relates to a process wherein, in the presence of hydrogen, phosphine is reacted with chlorine gas to prepare phosphorous trichloride, phosphorous pentachloride, or phosphorous tribromide.
Sodium hypophosphite is prepared by reacting tetraphosphorus, P4, with an aqueous solution of sodium hydroxide. This reaction produces a solution of about 50 wt % sodium hypophosphite, about 25 wt % sodium phosphite, and about 25 wt % of a gaseous mixture of phosphine and hydrogen in a 1:1 molar ratio. (Other alkali metal or alkaline earth metal hypophosphites can be made in the same manner.) At the present time, the practice is to burn the phosphine-hydrogen mixture in air followed by scrubbing in water to produce phosphoric acid. However, the resulting phosphoric acid has about the same value as the P4 starting material, so no value is added by these additional steps.
We have discovered that the phosphine-containing byproducts in the sodium hypophosphite reactor can be reacted with chlorine to produce phosphorous trichloride and hydrogen chloride without a significant loss of chlorine due to reaction of the chlorine with hydrogen. We have found that this reaction can be made to produce almost entirely phosphorous trichloride as the phosphorous-containing product under certain reaction conditions, despite the presence of hydrogen. Since the reaction is exothermic, it can be run adiabatically and continuously. Cooling is not needed unless the adiabatic temperature rise results in temperatures over 250xc2x0 C.
We have also found that air or oxygen can be introduced into the reactor downstream of the phosphine-chlorine reaction zone to stoichiometrically produce POCl3 as the virtually pure product. At the temperatures used in this invention, the oxygen will not react with the hydrogen that is present.
The starting material for the process of this invention is phosphine, PH3. Phosphine is a gas which typically contains small amounts of its dimer, diphosphine, P2H4. The invention is limited to mixtures of phosphine with hydrogen because the most commercially important process from which the phosphine is obtained necessarily includes hydrogen as a co-product. In that process, the hydrogen-phosphine mixture is obtained from a sodium hypophosphite reactor, in approximately a 1 to 1 molar ratio, but the invention is also applicable to mixtures of phosphine and hydrogen at molar ratios between 0.05 to a 0.95. Because water reacts with phosphorous trichloride to form phosphorous acid, the phosphine should be moisture free (i.e., less than 5 ppm of water and preferably less than 1 ppm water). The phosphine can be dried by passing it through a partial condenser and a suitable drying agent or moisture removal procedure, such as a molecular sieve, as is well known in the art.
The dry phosphine is reacted with chlorine gas according to the equation:
PH3+3Cl2xe2x86x92PCl3+3HCl.
Bromine gas can also be used to produce phosphorous tribromide and that reaction is also part of this invention. However, chlorine gas is preferred as phosphorous trichloride is more important commercially. Since the chlorine reacts with the phosphine in a 1:3 phosphine:chlorine ratio, less than a stoichiometric amount of chlorine will leave unreacted phosphine and more than a stoichiometric amount of chlorine will produce byproducts such as PCl5. Therefore, the molar ratio of chlorine to phosphine should be between 3 and about 3.1 and is preferably as close to stoichiometric as possible. If the desired product is phosphorous pentachloride the molar ratio of chlorine to phosphine should be between 5 and about 5.1.
The reaction can be performed at any temperature (e.g., room temperature) up to about 250xc2x0 C. At higher temperatures hydrogen that is mixed with the phosphine reacts with chlorine to form HCl and at lower temperatures the reaction is slow. Thus, it is preferable to perform the reaction at about 50 to about 125xc2x0 C. We have found that if excess chlorine is not present, i.e., the amount of chlorine is not in excess of the amount that will react stoichiometrically with the phosphine (3 moles of chlorine, Cl2, per mole of phosphine) a temperature range of about 50 to about 70xc2x0 C. will result in nearly 100% conversion of the phosphine to phosphorous trichloride and the HCl produced will not be much in excess of 3 moles per mole of phosphine. If excess chlorine is present, higher temperatures (125 to 250xc2x0 C.) are needed to obtain high yields phosphorous trichloride without the production of phosphorous pentachloride in the final product. The reaction can be started at room temperature and, since it is an exothermic reaction, the temperature can be permitted to increase adiabatically as long as the temperature does not exceed 250xc2x0 C. The reaction proceeds to completion at atmospheric pressure.
The products of the reaction, a phosphorous halide and hydrogen chloride, are separated by first condensing the phosphorous halides (phosphorous trichloride, b.p.=75.5xc2x0 C., phosphorous pentachloride, b.p.=162xc2x0 C., and phosphorous tribromide, b.p.=172.9xc2x0 C.) and then passing the non-condensible hydrogen chloride and unreacted hydrogen through a water scrubber to form hydrochloric acid, which can be sold or used in other chemical reactions. The hydrogen can be vented, burned, sold, or used in other reactions. Phosphorous trichloride and phosphorous tribromide are used to make phosphite esters, herbicides, and other chemicals. Phosphorous pentachloride is used to make pharmaceuticals and other specialty uses such as a chlorinating agent.
In an optional additional step, the phosphorous trichloride or phosphorous tribromide products can be reacted with oxygen to form phosphorous oxychloride, POCl3 or phosphorous oxybromide, POBr3 respectively. This reaction can be performed in situ without further isolation or purification of the phosphorous trichloride or phosphorous tribromide, but, to avoid making phosphoric acid, it must be performed after the phosphorous trichloride or tribromide have been formed. While oxygen gas can be used for this reaction, it is preferable to use air as it is less expensive and works as well. Phosphorous oxychloride and phosphorous oxybromide are easily separated liquids which are used as intermediates in making herbicides, plasticizers, and other products of commercial significance.